Various monitoring systems for detecting the intrusion of a foreign object in a body of water are known in the art. Such systems typically utilize one of three monitoring techniques: the measurement of water displacement; the detection of transient wave motion using a hydrophone or modified hydrophone; or detection of the amplitude of a sound wave generated by one transducer in the apparatus and received by another.
A system based upon water displacement is disclosed in U.S. Pat. No. 4,189,722 to Lerner, which describes a monitoring apparatus for detecting an intrusion into a pool based on a change in water level. This system employs a hydraulic cavity, partially filled with water, which maintains the average pool water level. The cavity has an upper frequency cut-off point that is low enough not to respond to surface wave action or sudden disturbances. When a large object intrudes into the pool, the water level in the cavity rises in proportion to the water displaced by the intruding object. The hydraulic cavity is a resonant cavity, and its resonant frequency change is detected and, upon exceeding some threshold level, an alarm is sounded. An inherent deficiency in this water displacement approach is that, particularly with very large pools, the water level displacement caused by the intrusion of a relatively small foreign object may be slight and thus may not be detected by the apparatus.
Monitoring devices which use a hydrophone or other device to detect transient wave motion are disclosed in U.S. Pat. Nos. 4,604,610, 4,853,691, 4,533,907 and 4,571,579 and are illustrative. U.S. Pat. No. 4,604,610 to Baker et al. discloses an apparatus which utilizes a submerged hydrophone, sensitive only to the vertical component of underwater wave motion, to detect transient wave motion caused by the intrusion of an object into a pool. Because the apparatus is dependent upon transient wave motion, it may fail to detect the intrusion of a small child or incapacitated individual that smoothly slides into the pool and causes little or no transient wave motion. U.S. Pat. No. 4,853,691 to Kolbatz is directed to a monitoring system that detects transient waves via either a switching element or a microphone. The Kolbatz system may fall to detect a small child or incapacitated person who falls into the pool and causes little or no transient wave motion. On the other hand, this system may generate a false alarm in response to transient wave motion caused by high winds.
U.S. Pat. No. 4,533,907 to Thatcher discloses an alarm system based upon a modified hydrophone to detect underwater transient wave motion. This system employs a long tube that is vertically immersed in the body of water being monitored, thereby trapping a small air cavity at the top of the tube. Underwater wave motion causes a fluctuation of the air pressure in the air cavity. This air pressure fluctuation is detected by a piezoelectric device, converted to an electrical signal, amplified and compared to a threshold value. If the fluctuation exceeds the threshold, an alarm is triggered. Again, this passive approach runs the risk of failing to detect the presence of a person who slides into the pool and does not cause a significant underwater wave front. A similar deficiency is present in the system disclosed in U.S. Pat. No. 4,571,579 to Wooley. The Wooley system employs a high Q hydrophone that is selectively sensitive to sound waves at its resonant frequency. Here, a transducer module immersed in the water has a resonant cavity that has an object inside it capable of freely being agitated by underwater disturbances.
The problems caused by transient motion detecting devices and the potential risk of failing to detect persons who generate little or no underwater wave activity have been remedied somewhat by monitoring devices which employ active sound navigation and ranging techniques and various configurations of transducers such as U.S. Pat. Nos. 4,747,085 and 4,932,009. U.S. Pat. No. 4,932,009 to Lynch, for example, describes a monitoring apparatus with multiple transmitters and receivers set up in a grid-like fashion such that each transmitter sequentially signals its corresponding receiver to establish a plane of detection. An object intruding in this plane will block one of the transmitter receiver pairs and either alter the strength of the sound wave passing between the transducers or completely inhibit its detection. The decrease in strength or complete failure to detect a particular sound wave emitted by the apparatus indicates an alarm condition. Because the apparatus operates by detecting a sound wave actively generated by the apparatus, it detects the intrusion of an object regardless of whether that object generates any type of transient wave motion. However, the device can be extremely costly. Each transmitter receiver pair is based upon underwater sound propagation using ceramic based transducers. As such, a large number of transducers and configurations thereof would likely have to be installed. This could make the cost prohibitive for many pool applications.
U.S. Pat. No. 4,747,085 describes a two transducer system. One transducer converts a continuous electrical signal into an underwater sound wave and the second transducer receives a return continuous sound wave. The theory behind the apparatus is that a continuous sound signal flooding a pool of given geometry will return a unique continuous signal signature. Once this signature is established, any intrusion by a foreign object will disturb or modulate it. This type of system has several deficiencies. First, when a pool is flooded with sound waves, the sound propagates in all directions, reflecting off of side walls and recombining with original waves and other reflections. When original attenuated waves recombine with other reflected waves, the original and reflected waves cancel each other thereby enabling dead zones to develop in the water. These dead zones have no sound waves. Therefore, any object that happens to fail in a dead zone fails to cause a modulation of the signature, and thus is not detected. Secondly, continuously driving a transducer at the power level needed causes very large power dissipation, which is not conducive to battery backup operation and also increases cost.